Florida water tank won't flush

I've had similar things happen when we were about to go away. The funniest looking back was when I thought I'd check the tyres pressures while I was waiting for the others to come out the house.  I put the gauge on the valve and the pressure was fine but when I removed it the valve stayed open!! Panic no matter what I did it would not close. I thought I'd take the dust cap off another tyre and use this to stem the flow but none of them were actually air tight and the tyre was visibly going down.  In the end I dashed out to a car in the street and 'borrowed' one, it must have looked hysterical. 

Another time I got about 40 miles from home and pulled into a garage to fill up to discover I did not have a key for the fuel cap on the key ring and we were already on empty!! Managed to get home for the key but it was not a good start to a holiday with the kids.